APO Upgrade connection to OLTP System management

Hi all, we are in process to upgrade APO 5.1 to 7.2 and the process as of now if going very well. My only concern is when I will reach the production system. I am not sure about how to manage the OLTP (R3) connection / CIF queue when I will reach the downtime Phase of my SCM upgrade to keep the consistency between the R3 system and the SCM system. We discuss about these scenarios when we will reach the downtime phase :

Scenario # 1:

- We shutdown the R3 system until the SCM is upgraded.

Scenario # 2 :

- We stop the CIF queue (Inbound / Outbount) in SCM and in R3 systems.

- We deactivate the integration model in R3 too ...

Scenario # 3 :

- We cancel the background processing in R3 system and we lock the connection to all users until the migration is done !

Scenario # 4 😔 :

- We leave the system as is and we clean all the queue in R3 system until the connection is UP between SCM and R3 and we run a consistency check to consolidate everything.

Any help will be really appreciate and if somebody else have the "Scenario # 5 ", I am fully open 😊

Thanks for help